Honeycombed sintered ceramics are of current interest as substrates for catalytically active substances used in apparatus for the conversion of hydrocarbon, carbon monoxide and nitrogen oxides from automobile exhaust gas. Other uses for such inorganic substrates outside of the auto industry are also recognized.
Several properties are required of ceramic substrates if they are to survive operation in automobile catalytic reactors. Two of these properties were the subject of copending U.S. patent applications Ser. Nos. 412,862 and 412,863 (now U.S. Pat. No. 3,950,175), both filed Nov. 5, 1973. The two copending applications relate to substrates consisting primarily of polycrystalline cordierite, the former concerned with very low thermal expansion compositions and the latter with batch materials stimulating significant coarse porosity in the sintered ceramic. The present invention pertains to the same basic cordierite compositions and to methods of producing large honeycombed substrates without incurring significant cracking of the ware during thermal processing. Some investigators have previously suggested the use of calcined clays in cordierite compositions but generally for the purpose of shrinkage or heat shock resistance. U.K. Pat. Nos. 886,718 and 888,227 suggest prefired clay for the latter purpose while allowing up to 70 and 80 percent raw clay in the material batches. Cracking behavior as a function of raw clay in the batch was not envisioned and therefore no limit was established for the raw clay component making up the raw material batches for large ceramic bodies.
Standard substrate sizes for insertion in catalytic reactors are at least 3 inches and sometimes 5 inches in diameter. The length of the substrates vary, but from a production standpoint it would be desirable to fire substrates 16-36 inches long and then cut them to the useful length (usually about 3 inches long). Typically, however, compositions necessary for optimizing other properties (including thermal expansion and porosity) were prone to cracking when made into the elongated (16-36 inches) substrates (in the art and herein also referred to as "logs") and fired. Exceptionally slow firing schedules could be used although this is not an economically desirable result.